1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention particularly relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (the transistor is also referred to as thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device).
In accordance with an increase in integration of a circuit, a transistor is miniaturized. When the transistor is miniaturized, there is a possibility that electrical characteristics of the transistor, such as the on-state current, the off-state current, the threshold voltage (hereinafter, referred to as threshold value), and the subthreshold swing value (S-value) degrade. When the channel length is decreased, an increase of the off-state current, an increase of the shift of the threshold value, and an increase of the S-value generally occur. When the channel width is decreased, the on-state current is reduced.
As the current at which the voltage between a gate and a source is 0 V (Icut current) is small, the power consumption of the circuit is lowered and the leakage of charge is suppressed. In an n-channel transistor, the Icut current is increased when the threshold value of the transistor shifts on the negative side. Thus, in order to reduce the Icut current, the threshold value needs to be shifted on the positive side. However, when the threshold value is excessively shifted on the positive side, the operating voltage of a circuit is too high, and the power consumption is increased. The threshold value of the transistor needs to be controlled to suppress the leakage of charge and lower the power consumption of the transistor. In description of Patent Document 1, charge is trapped in a gate insulating film to control the threshold value.
An insulating film containing silicon and nitrogen is known as an effective trap film, which is a material capable of trapping both electrons and holes. Thus, by injecting either of the carriers in accordance with a desired threshold value, the threshold value can be adjusted to be on either the positive side or the negative side. In addition, a film with high trap density can be formed by adjusting the film formation conditions (see Patent Document 2), and a film with high retention capability can be formed (see Patent Document 3).